With the increase in variety of electronic apparatuses for portable use such as cellular phones, portable audio-video equipment, and the like in recent years, there is continued advancement in multi-functional capability as well as reduction in size, thickness and weight. Movable contact units of a type having a plurality of movable contacts retained on an insulating plastic film are used in large number as switches for control panels of the electronic apparatuses, as they can be made thin, and provide good tactile response as well as stable electrical contact. The movable contacts are made of electrically conductive metal sheets formed into a dome-like shape.
Since these apparatuses are being carried and used anywhere, the switches are operated so frequently that they require movable contact units of high durability and stable tactile response in the switching operation.
Referring now to FIG. 7, a description is provided hereinafter of a conventional movable contact unit of such kind used for the switch of a control panel.
In the conventional movable contact unit, as shown in FIG. 7, a plurality of dome-like movable contacts 3 are arranged independently with respect to one another, with their upper surfaces retained by adhesion of adhesive layer 2 formed on an underside surface of flexible insulating base film 1 having an external shape formed into a predetermined shape.
Movable contacts 3 are sandwiched between separator 4 made of an insulating film having a surface treated with release agent and base film 1. Separator 4 is held adhered to adhesive layer 2 on base film 1 in a manner that it covers the entire underside surface of base film 1.
As separator 4 is placed in close adhesion to base film 1 in a manner to completely cover the outer peripheries of movable contacts 3, it prevents corrosion of movable contacts 3. In addition, separator 4 also prevents adhesive layer 2 on base film 1 from sticking to other surfaces and gathering foreign objects unintendedly during transportation and in the storage.
The conventional movable contact unit further has small cylindrical parts 5A, which are fabricated by die-cutting a plastic film into a cylindrical shape, and bonded to base film 1 by adhesive resin 5B composed of epoxy acrylate, i.e. a kind of resin curable by ultraviolet rays. Each of cylindrical parts 5A and adhesive resin 5B compose press-down projection 5.
FIG. 8 shows a structure of the conventional movable contact unit in the actual application. After separator 4 is peeled off, base film 1 bearing movable contacts 3 is attached to wiring board 7 using adhesive layer 2 so that movable contacts 3 are aligned in a manner to face with their respective sets of stationary contacts 6 (6A and 6B).
FIG. 8 depicts wiring board 7 with the movable contact unit attached to it. The center portion of movable contact 3 confronts the respective one of central stationary contacts 6A, and the peripheral edge of movable contact 3 lies on the corresponding outer stationary contact 6B, so that each movable contact 3 and the corresponding stationary contacts 6 compose a single switch. In addition, actuator button 8 of the apparatus is arranged above press-down projection 5.
A switch for control panel using the conventional movable contact unit constructed as above operates in a manner which will be described now with reference to FIG. 9.
When actuator button 8 is depressed in a direction shown by an arrow in FIG. 9, the depressing force is given on the upper surface of cylindrical press-down projection 5, which is in contact to underside surface 8A of actuator button 8. The depressing force is thus applied to the center portion of movable contact 3 through press-down projection 5 and base film 1.
When the depressing force exceeds an inflectional strength of movable contact 3 of the dome-like shape, it bends into an inverted shape with a click-feeling, and an underside surface in the center of movable contact 3 comes in contact with central stationary contact 6A. This makes an electrical connection between central stationary contact 6A and outer stationary contact 6B on wiring board 7 via movable contact 3.
When the depressing force is removed, movable contact 3 regains its original shape by an elastic restoring force of its own. This separates the underside surface in the center of movable contact 3 from central stationary contact 6A, to restore the state of electrical isolation between the central stationary contact 6A and the outer stationary contact 6B of wiring board 7. By allowing the optimum click-feeling in the depressing operation, the above structure provides the control panel switch with good tactile response.
Due to the rapid-paced advancement in the multi-functional capability in addition to downsizing of portable apparatuses in the recent years, a number of functions assigned to a single switch increases, which consequently increases frequency of depressing operation of the switch. There is thus growing demand for improvement of durability to the switching operation.
In other words, the desire continues to increase for a movable contact unit designed to increase strength of adhesion of press-down projection 5 to base film 1 with steadiness of the tactile response even after repeated operations.